The flying delivery truck: Boeing 737 Classic freighters

Even though they had always lived in the shadow of their big brothers – the 747F, 767F and 777F – Boeing’s narrowbodies were always a common sight in the air cargo world. Indeed, the daily intra-European flow of goods has (mostly) been in the hands of converted 737s and 757s for years now, quietly moving everything from Amazon orders to vital public healthcare supplies. Despite having made the limelight only due to the corona crisis, they’ve been regular guests at Zagreb for quite some time – which is a perfect opportunity to show one of them a little bit of love…

However, having caught the public’s attention during the lockdown – when they were frequently the only traffic in the air – it is necessary to first separate them from improvised freighters repurposed from unused passenger airliners. Grouped together under the generic term Simplified Package Freighters, these modifications were strictly of a temporary nature, and usually involved just removing the seats and cabin dividers and installing freight safety nets – with the aircraft themselves structurally remaining as they were. Because of this, they were legally limited to transporting solely package goods of limited contents – such as masks and inert medical equipment – which often had to be loaded by hand through the passenger doors.

“Our” freighters however had been though what’s called a Passenger-to-Freight (P2F) conversion, which involves extensive and permanent alterations to the aircraft structure and systems. As a consequence they can no longer be used to transport passengers – at least not without being deconverted, which is not financially viable.

Of the several conversions available for the 737 Classic family, the most successful – and most often seen at Zagreb – is the Special Freighter (SF) by Aeronautical Engineers Inc (AEI). The recipe for this sort of thing is actually quite simple: find a 300 or 400 series in good shape and with enough usable life left, paid off and without bank dues, cheap and easily available because it is “old hat” – and then turn it as simply as possible into a flying truck. The intention is to get a complete solution that can carry a high number of standardized freight containers (Unit Load Device, ULD) – and that is cheap enough AFTER conversion to appeal to smaller operators that frequently fly for big logistics companies. Low acquisition and conversion prices are a must also because of higher operational costs compared to more modern aircraft, including higher fuel consumption and more expensive maintenance.

Despite these minimalist principles, the list of alterations to the aircraft is quite long – to satisfy all legal, operational and financial requirements – and includes:

installation of a 3.55 x 2.23 m cargo door on the left side
installation of a standalone hydraulic system for the cargo door (powered from the aircraft’s electrical network)
reinforcement of the aircraft structure and replacement of fuselage skin panels around the cargo door to compensate for reduce airframe stiffness
reinforcement of floor beams in the back of the cabin (on many aircraft types lighter than the beams further forward)
installation of suitable guiderails, rollers and cargo safety net mounting points
installation of a new cabin lighting system
replacing cabin windows with plugs (except in on the wing emergency exits) and fitting of protective panels to the interior of the fuselage
relocation the „black boxes” to increase the height of the ceiling in the back of the cabin
installation of a new fire protection system in the cabin¹
and installation of a fireproof safety barrier between the cabin and crew areas

As well as the complete interior, the conversion also entails the removal of systems that were needed for passenger operations only, such the aft lavatory, its plumbing and tanks – as well as the emergency slides on the rear doors. Since freighters of this type are usually used on flights of at most two hours, „creature comforts” for the crew have been reduced to Spartan levels as well in order to reduce weight, price, complexity and maintenance costs. Forward of the safety barrier – located at the very end of the cabin, just aft of the forward passenger doors – the only things that remain are the lavatory, two or four seats for additional crew (depending on customer requirements), various emergency equipment and the control panel for the cargo door. In addition to the kitchen and potable water system, the forward slides can be removed as well, in which case the crew will evacuate through the cockpit exit.

The safety barrier also includes a small access door, allowing entry into the cabin in flight – though it is normally shut, since then it completes a hermetic seal between the cabin and crew areas. The normal underfloor cargo compartments remain available under the same conditions as before, and can continue to be used for transporting smaller packages.

What you get in the end is summed up in the following table (contained sizes are expressed as length x width x height):

	737-300SF 9 ULD configuration	737-300SF 10 ULD configuration	737-400SF Standard Gross Weight	737-400SF Maximum Gross Weight
Capacity:	8 containers 318x224x208 cm + 1 container 318x224x163 cm	8 containers 318x224x208 cm + 2 containers 224x135x163 cm	10 containers 318x224x208 cm + 1 container 224x135x163 cm
*Payload:**	~ 19,500 kg		19,600 kg	21,400 kg
Volume – cabin:	109 m³		129 m³
Volume – underfloor:	27.5 m³		35.5 m³

* depends on the aircraft’s basic empty weight

At the time of writing, AEI had delivered a total of 137 conversions for the 737 Classic family², of which 17 were 300s and 120 400s. The disproportionate success of the 400 series – despite fewer having been made, 486 vs 1113 – can be explained by its better benefit-cost ratio in cargo operations, which are generally a combination of short distances and high freight volumes. Indeed, the European market alone – dominated by many national borders and terrain not often favorable to road transport – had taken more than 50% of all the conversions made so far.

In Croatia itself, the most frequently seen examples belong to the continent’s three dominant SF operators, flying in mostly as replacements for the regular DHL/EAT 757 on the weekday Leipzig-Zagreb run via Bergamo or Malpensa:

ASL Airlines (Hungarian branch): 6x 400SF (average age 28 years)
Cargo Air (Bulgaria): 7x 400SF (27.2 y) + 3x 300SF (31.7 y)
Swiftair (Spain): 9x 400SF (28 y) + 1x 300SF (33.6 y)

Very occasionally, Zagreb also sees examples from Bluebird Nordic (ex. Bluebird Cargo, 5x 400SF, 27.5 y) and ASL branches in other countries (most notably Ireland, 8x 400SF, 26.9 y), usually as substitutes for the aforementioned Big Three.

Freight Trek: The Next Generation

While the 300 and 400 series had shown themselves suitable for conversion specifically because of their availability, the air freight sector’s increasing hunger for more and more airplanes has drastically reduced both the size and quality of the remaining stock. In order to mitigate this growing problem, several companies had developed conversions for the early examples of the 737 Next Generation (700 & 800), which were expected to hit the market in high quantities and at excellent terms with the coming of the 737MAX. Currently available NG conversions include:

Israeli Aircraft Industries 737-700BDFS (Bedek Special Freighter, up to 10 ULDs) and 737-800BDSF (up to 12 ULDs)
AEI 737-800SF (up to 12 ULDs)
Boeing’s own 737-800BCF (Boeing Converted Freighter, up to 11 ULDs)
and, somewhat in a class of its own, the PEMCO 737-700FC (FlexCombi, a combined passenger-freight configuration with up to eight ULDs)

Essentially, all of these conversions are very similar to those for the 300 and 400 series, and entail the same sort of changes: a cargo door, reinforcements where necessary, a new fire protection system and removal of unnecessary equipment. Their future is now tied to the fate of the MAX, since its protracted grounding has forced many NG operators to keep their aircraft in service longer than planned. As of July 2020, the two biggest players in this field – AEI and Boeing – had delivered only five 800SFs and 34 800BCFs between them.

¹ because of its negative effects on human health, passenger cabins do not have an integral fire protection system; any fires are thus put out using portable fire extinguishers. By their very nature, cargo compartments however have to have purpose-built systems, which nowadays work by saturating the compartment with Halon gas that binds itself to free oxygen, therefore starving the fire (this is called a Class C Cargo Compartment).

On 737-sized aircraft, the cargo compartments – located under the cabin – are sufficiently small that they don’t require impractical amounts of Halon for extinguishing to be effective (and for saturation to be maintained until landing). On converted freighters though, the cabin volume is up to five times greater than the volume of the cargo compartments, making it unfeasible to carry a sufficient amount of Halon. Things are complicated further by the nature of the cargo itself, which is far more diverse in content and higher in mass, density and flammability than standard baggage, which increases problems by an order of magnitude and further ups any Halon requirements. Indeed, the above is one of the main factors limiting what can actually be transported on Simplified Package Freighters, as they lack any form of firefighting capability in the cabin.

Since the most effective solution is still to starve the fire of oxygen, on converted freighters it must be possible to hermetically seal off the entire cabin (hence the reason why the safety barrier door has to be closed). Valves are also installed that can cut off the supply of fresh air to the cabin on command – forcing the fire to quickly use up what oxygen remains and then die out (this is called a Class E Cargo Compartment). It is important to note that the bay is NOT depressurized in the process, since the pressure difference between different compartments inside the airplane could lead to structural damage to both the barriers, floor and airframe.

² alongside the 737 Classics, AEI offers conversions for several other types, including the 737-200 (20 completed), CRJ-200 (13), MD-80 (19) and 727 (224).